Transfer case with tubular output shaft

ABSTRACT

A light-weight transfer case is provided for implementation with a four-wheel drive vehicle. The light-weight transfer case includes a single-piece housing formed through either a lost-foam magnesium or die cast process. First and second output shafts are included which are formed from single-piece tubing through either a hydro-forming or swaging process. The first and second output shafts are lighter weight and maintain increased strength over traditional transfer case output shafts. A gear reduction unit is also included for establishing high, low and neutral speeds of the first and second output shafts. Furthermore, a mode selection device is included for selectively providing drive to either a single output shaft, in a two-wheel drive mode, or both the first and second output shafts, in a four-wheel drive mode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/278,140, filed Mar. 23, 2001.

FIELD OF THE INVENTION

The present invention relates generally to transfer cases for use infour wheel drive vehicles. More particularly, the present invention isdirected to a light-weight transfer case improving overall vehicle costand efficiency.

BACKGROUND OF THE INVENTION

As is known, the majority of four-wheel drive vehicles are equipped witha transfer case mounted to a multi-speed transmission for directingpower from the engine to all four wheels. To accommodate different roadsurfaces and conditions, many transfer cases are equipped with a modeshift mechanism which permits the vehicle operator to selectively couplethe non-driven wheels to the driven wheels for establishing a part-timefour wheel drive mode in addition to the two-wheel drive mode. As analternative, some transfer cases are equipped with a transfer clutchthat is passively or actively controlled in response to driveline slipfor automatically delivering drive torque to the non-driven wheels forestablishing an on-demand four-wheel drive mode. In addition, sometransfer cases are also equipped with a two-speed range shift mechanismfor permitting the vehicle operator to select between high-range andlow-range four-wheel drive modes.

Automobile manufacturers continuously strive to reduce vehicle weightand improve vehicle noise, vibration and harshness (NVH)characteristics. In particular, sport utility vehicles (SUV) enjoy asignificant portion of the overall vehicle market. The majority of theseSUV's provide a four-wheel drive mode and, therefore, are typicallyequipped with a transfer case. As part of the vehicle's driveline, atransfer case has significant influence on the NVH characteristics ofthe vehicle. For example, vibrations and excitations generated by thetransmission are transferred through the transfer case to front and rearpropshafts. Additionally, the transfer case itself can be a source ofNVH excitation.

On critical characteristic of four-wheel drive vehicles is the weight ofthe transfer case. Specifically, the shafts used in transfer cases aregenerally manufactured from solid forgings which are machined to formvarious gear segments, bearing and stop surfaces, as well as otherfeatures along the length of the shaft. Furthermore, traditionaltransfer cases include a multi-piece cast housing which includes atleast two housing sections that are bolted together for enclosing andsupporting the internal components. Because the housing sections arebolted together, each section requires a peripheral flange through whichthe bolts extend. In view of the recognized needs to reduce vehicleweight for improved fuel economy and to improve vehicle NVHcharacteristics, it is desirable to develop a light-weight transfer caseproviding improved NVH characteristics.

SUMMARY OF THE INVENTION

The present invention is directed to a transfer case for use in afour-wheel drive vehicle having improved weight and NVH characteristics.These improvements are provided by transfer case having tubular shaftsand a one-piece housing enclosed with end plates. To this end thetransfer case of the present invention includes a one-piece housingdefining first and second apertures and an opening, a first cover plateenclosing the first aperture of the housing and defining an opening, anda second cover plate enclosing the second aperture of said housing anddefining an opening. The transfer case also includes an input shaftextending through and rotatably supported in tje opening in the firstcover plate, a first output shaft driven by the input shaft andextending through and rotatably supported in the opening in the housing,a second output shaft extending through and rotatably supported in theopening in said second cover plate, and a mode clutch for transferringdrive torque from the first output shaft to the second output shaft.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of the present invention willbecome apparent to those skilled in the art from studying the followingdescription and the accompanying drawings in which:

FIG. 1 is a schematic representation of a drivetrain for a four-wheeldrive vehicle equipped with a light-weight transfer case according tothe present invention;

FIG. 2 is a sectional view of the light-weight transfer case of thepresent invention;

FIGS. 3A and 3B are partial sectional views of two shafts comparinggrain structure according to the principles of the present invention;and

FIG. 4 is a sectional view of an alternative construction for thelight-weight transfer case of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates generally to light-weight transfer casesfor use in four-wheel drive vehicles for providing drive torque androtational motion to front and rear drivelines. In particular, thelight-weight transfer case comprises components having reduced weightbut which retain required strength and stiffness properties. Theselight-weight components include a one-piece housing and tubular frontand rear output shafts. Additionally, as a result of the forming processused to manufacture the tubular output shafts, each can be tuned toreduce the noise, vibration, and harshness (NVH) characteristics of thetransfer case.

With reference to FIG. 1, an exemplary motor vehicle drivetrain 10 of atype suitable for use with the present invention is schematically shown.Drivetrain 10 has a pair of front wheels 12 and a pair of rear wheels 14drivable from a source of power, such as an engine 16 through atransmission 18. It is foreseen that transmission 18 may be either ofthe automatic or manual types commonly known in the art. In theparticular embodiment shown, drivetrain 10 is a rear wheel drive systemwhich incorporates a light-weight transfer case 20 that is operable toreceive drive torque from transmission 18 for normally driving rearwheels 14 in a two-wheel drive mode of operation. Additionally,light-weight transfer case 20 is adapted to permit a vehicle operator toselectively transfer drive torque to front wheels 12 for defining afour-wheel drive mode of operation.

Typically, front and rear wheels 12, 14 have a common rolling radius andare part of front and rear wheel assemblies 24, 26 which, in turn, areconnected at opposite ends of front and rear wheel axle assemblies 28,30, respectively. A front differential 32 is mechanically coupledbetween front axle assembly 28 and a front prop shaft 36 such that frontwheel assemblies 24 are driven by front prop shaft 36 when light-weighttransfer case 20 is operating in the four-wheel drive mode. Similarly,rear axle assembly 30 includes a rear differential 34 coupled in drivenrelationship to a rear prop shaft 38 for driving rear wheel assemblies26. It is to be understood that the orientation of drivetrain 10 ismerely exemplary in nature and that the drivetrain could be reversed fornormally driving the front wheels 12 in the two wheel drive mode.

With reference now to FIGS. 1 and 2 rear prop shaft 38 is adapted to beconnected to a rear output shaft 40 of light-weight transfer case 2 ovia a suitable rear coupling 42. Similarly, front prop shaft 36 isadapted to be connected to a front output shaft 44 via a suitable frontcoupling 46. A transmission output shaft (not shown) couplestransmission 18 to an input shaft 48 of light-weight transfer case 20for supplying power thereto. Transfer case 20 is shown to include aone-piece housing 50. Housing 50 is preferably cast from aluminum ormagnesium utilizing a lost foam casting process. Housing 50 includes afirst aperture 52 and a second aperture 54, each sized to permitassembly of various components into an internal chamber 56. As describedhereinbelow, input shaft 48 and rear output shaft 40 rotatably supportvarious components within chamber 56 and are themselves rotatablysupported at one end by housing 50 and at an opposite end by a firstcover plate 58 which encloses first aperture 52 of housing 50.

First cover plate 58 includes a plate segment 58 a interconnecting aninner annular hub 58 b and an outer annular hub 58 c. Outer hub 58 c ofcover plate 58 is seated in first aperture 52 and includes a ring seal60. As seen, a stop face 62 of outer hub 58 c abuts a radial shoulder 64formed in first aperture 52. First cover plate 58 is held in positionwith stop face 62 against shoulder 64 by a circlip 66. A bearingassembly 68 is retained between inner hub 58 b of first cover plate 58and input shaft 48 to facilitate rotation of input shaft 48 relative tohousing 50. A seal assembly 70 provides a fluid-tight rotary sealbetween input shaft 48 and first cover plate 58.

Rear output shaft 40 is a tubular component aligned on the longitudinalaxis of input shaft 48 and has a small diameter pilot segment 72 and alarge diameter shaft segment 74. Rear output shaft 40 is preferably madeusing a swaging process with a tubular member having the diameter ofshaft segment 74 drawn or elongated at one end to form pilot segment 72and a tapered transition segment 75 therebetween. Pilot segment 72 isrotatably supported by a bearing assembly 76 in an axial bore 78 ofinput shaft 48. An end plate 79 encloses the terminal end of pilotsegment 72. A seal cap 80 provides a seal between bore 78 of input shaft48 and an internal chamber 82 of rear output shaft 40. Throughbores 83in end plate 79 and pilot segment 72 permit hydraulic fluid in chamber82 to lubricate various rotary components through which lubricant flows.Hydraulic fluid is supplied to chamber 82 from a shaft-driven pump 84which draws fluid from a sump provided with chamber 56 of housing 50.

The axial position of pilot segment 72 of rear output shaft 40 ismaintained relative to input shaft 48 via a thrust washer 86 whichaccommodates relative rotation therebetween. The opposite end of rearoutput shaft 40 is shown with end portion of shaft segment 74 extendingthrough a first cylindrical opening 88 formed in housing 50 androtatably supported therein by a bearing assembly 90. A rotary sealassembly 92 is also shown to extend between shaft segment 74 of rearoutput shaft 40 and first opening 88. Internal splines 93 are formed(i.e., rolled) in the open end of rear output shaft 40 and are adaptedto receive an externally splined component of rear coupling 42.

With continued reference to FIG. 2, front output shaft 44 is shown to bea shaped tubular component having a first end segment 94, a second endsegment 96, and a central sprocket segment 98. First end segment 94 iscylindrical and is enclosed by an end wall 100. First end segment 94 isshown to be retained in a boss segment 102 of housing 50 and rotatablysupported therein via a bearing assembly 104. Second end segment 96 isalso cylindrical and is mounted by a bearing assembly 106 and a rotaryseal assembly 108 to a second cover plate 110. Second cover plate 110encloses second aperture 54 of housing 50. Second cover plate 110includes a radial plate segment 110 a and an annular hub segment 110 b.Plate segment 110 a of second cover plate 110 is seated in secondaperture 54 and includes a ring seal 112. A stop face 114 on platesegment 110 a abuts a shoulder surface 116 on housing 50 while a circlip118 secures second cover plate 110 to housing 50. Second end segment 96of front output shaft 44 extends through an opening 120 in second coverplate 110 and is adapted for connection to front prop shaft 36 viacoupling 46. Specifically, second end segment 96 has internal splines121 formed therein adapted to receive externally splined component offront coupling 46.

Input shaft 48 has an input sun gear 122 of a planetary gearset 124formed integral therewith. Planetary gearset 124 is a speed reductionapparatus operable for defining high and low speed ratios relative toinput shaft 48. It will be understood that planetary gearset assembly124 is merely exemplary of a suitable two speed gear apparatus for usein light-weight transfer case 20. Sun gear 122 is shown meshed with aplurality of planet gears 126. Each planet gear 126 is rotatablyjournalled on a pin 128 supported in a planetary carrier 130. Planetarycarrier 130 includes fore and aft ring members 132 and 134 securedtogether by bolts (not shown). Planet gears 126 also mesh with anannulus gear 136 that is non-rotatably mounted to housing 50.Specifically, annulus gear 136 is retained against rotational movementby a plurality of radially extending tabs 138 which are received incorresponding longitudinal grooves formed in housing 50. Annulus gear136 is additionally retained against axial movement away from a stopshoulder 140 formed in housing 50 by retention lugs 142 formed on firstcover plate 58

Transfer case 20 also includes a range clutch 150 and a shift mechanism152. Range clutch 150 includes a range sleeve 154 supported via a splineconnection 156 for rotation with rear output shaft 40 and axial movementthereon between three distinct positions. In the first position, denotedby a “H” position line, external clutch teeth 158 on range sleeve 154are meshed with internal clutch teeth 160 formed on input shaft 48,thereby establishing a direct or high-range drive connection betweeninput shaft 48 and rear output shaft 40. In a second position, denotedby a “L” position line, external clutch teeth 158 on range sleeve 154are meshed with internal clutch teeth 162 formed on aft ring 134 ofplanetary carrier 130, thereby establishing a reduced or low-range driveconnection between input shaft 48 and rear output shaft 40. Finally, inits third position, denoted by a “N” position line, a non-driven neutralmode is established with range sleeve 154 disconnected from both inputshaft 48 and carrier 130 such that no drive torque is transferred frominput shaft 48 to rear output shaft 40. Spline connection 156 includesexternal splines 164 that are roll formed on an external surface 166 ofshaft segment 74.

Shift mechanism 152 is operable for selectively moving range sleeve 154between its three distinct positions. Shift mechanism 152 includes arange fork 170 journalled for axial movement on a shift rail 172 andhaving a C-shaped fork setment 174 retained in a peripheral groove 176formed in range sleeve 154. One end of shift rail 172 is retained in aclosed cylindrical boss 178 formed in housing 50 while its opposite endis retained in a cylindrical bore 180 formed in housing 50. An end cap182 is shown to enclose bore 180. A cam follower 184 secured to atubular section 175 of range fork 170 is retained in the helical groove186 of a cam 188 that is shown secured to drive shaft 190. One end ofdrive shaft 190 is retained in a closed boss 192 formed in housing 50and its opposite end extends out of a bore 194 also formed in housing50. The second end of drive shaft 190 is coupled to a geartrain of anelectric motor assembly 196.

With continued reference to FIG. 2, a mode clutch 200 is provided toselectively shift light-weight transfer case 20 between a two-wheeldrive mode and a four-wheel drive mode. Mode clutch 200 includes a hubmember 202 that is splined to rear output shaft 40 and an axiallymoveable mode sleeve 204 shown in a central disengaged or two-wheeldrive mode (2WD) position. Mode sleeve 204 is formed with internalspline teeth 206 which are in constant axial sliding engagement withexternal spline teeth 208 on hub member 202. A mode fork 210 is coupledto mode sleeve 204 for permitting axial movement of mode sleeve 204 viaselective actuation of shift mechanism 152. A tubular section 211 ofmode fork 210 is secured via pin 212 to rail 172 and is biased by aspring 214 such that a cam follower 216, mounted to mode fork 210, isbiased against an outer surface 218 of cam 188. Thus, mode sleeve 204may be selectively shifted from the two-wheel drive mode (2WD) positionshown to a four-wheel drive mode (4WD) position whereat internal splineteeth 206 drivingly engage external spline teeth 220 formed on a chaincarrier 222. Chain carrier 222 is journalled on shaft segment 74 of rearoutput shaft 40 and also includes a drive sprocket 224. Drive sprocket224 engages a chain 226, shown in dashed lines, which is coupled to adriven sprocket 228. Driven sprocket 220 is secured to or an integralportion of sprocket segment 98 of front output shaft 44. It should alsobe noted that front output shaft 42 is formed from tubular materialsimilarly to rear output shaft 40, as discussed above. For example, anexpandable mandrel tool may be inserted into a tubular work piece andexpanded to form the shaped configuration of front output shaft 44. Assuch, front output shaft 44 incorporates the weight and NVH advantagesresulting from the tubular forming process.

A mode selector 230 permits the vehicle operator to select any one ofthe available two-wheel and four-wheel high-range and low-range drivemodes. A mode signal from mode selector 230 is sent to a controller 232which sends the appropriate electric control signal to motor assembly196 to control rotation of cam 188. As will be understood, the contourof helical cam track 186 associated with range fork 170 and the contourof cam surface 218 associated with mode fork 210 acts to coordinatemovement of range sleeve 154 and mode sleeve 204 to establish thevarious drive modes in response to the rotated position of cam 188. Asis well know, the mode clutch 200 can be replaced with a passivecoupling (i.e., viscous coupling, geared traction unit,gerotor-activated clutch, etc.) or an electronically controlled activecoupling (i.e., power-operated transfer clutch) as known in the art.

As a result of the various components which rear output shaft 40 mustsupport and the rotatable interface between rear output shaft 40 andhousing 44 and first cover plate 52, rear output shaft 40 requiresvarious diameter changes along its length. As such, a stepped segment 75is formed between larger diameter segment 74 and smaller diametersegment 72. Output shaft 40 is a formed tube which offers significantweight and strength advantages over traditional forged shafts. The tubecan be formed through any of several forming processes known in the art.For example, hydro-forming or swaging could be used. Because the tube isformed, as opposed to turned, the metal's grain structure continuouslyflows along the entire length. As a result, the strength of output shaft40 is maximized, while minimizing the amount of material required (i.e.in a cross-section a formed tube has thinner walls than an analogousturned bar shaft). With particular reference to FIGS. 3A and 3B anexemplary cross-section of each of a traditionally machined shaft 40Aand a formed shaft 40 are shown, respectively. Machined shaft 40A andformed shaft 40 each include a step 75A and 75, respectively. In formingstep 75A of machined shaft 40A, excess material is cut away. Thismaterial is represented by the shadowed section labeled “A”. As such,the grain structure of machined shaft 40 is discontinuous at step 75. Incontradistinction, the grain structure of formed shaft 40 is continuousthrough step 75, resulting in improved strength. Additionally, the wallthickness ‘X₁’ of machined shaft 40A is much thicker than the wallthickness ‘X₂’ of formed shaft 40.

Another significant advantage of formed shaft 40 is the unique abilityto ‘tune’ it for particular excitation frequencies. Resonant frequenciesthrough driveline components, including transfer case shafts, can resultin significant NVH problems. To minimize these problems, shaft 40 can beformed to include additional steps or other features which effectivelytune shaft 40 out of the excitation range. In comparison, traditionalshafts require increased mass or additional dampers for curing thesetypes of NVH problems. However, increased mass results in increasedweight and dampers increase both cost and weight, as well as increasingpackaging complexity within the transfer case. Obviously, the teachingsrelative to shaping a tubular rear output shaft 40 are also applicableto front shaft 44.

Referring now to FIG. 4, transfer case 20′ is shown to now be equippedwith a modified rear output shaft, identified by reference numeral 40′.Rear output shaft 40′ is a two-piece assembly having a shaft segment 74′and a pilot segment 76′. Pilot segment 76′ is secured (i.e., welded) toa forward end of shaft segment 74′. This arrangement of a two-pieceshaft 40′ eliminates the need to perform a shaft forming operation. Inaddition, a modified front output shaft 44′ is shown installed intransfer case 20′. As shown, front output shaft 44′ has a tubular shaftsegment 250 to which drive sprocket 228′ is secured (i.e., welded,splined, etc.) for common rotation. Tubular shaft segment 250 has auniform wall thickness across its length such that bearing assemblies104 and 106 are supported thereon. Radial plate segment 110A′ of secondcover plate 110′ has be slightly modified to accommodate retention ofseal assembly 108 on shaft segment 250.

Finally, a cylindrical insert 254 is secured (i.e., welded) in theforward open end of shaft segment 250 and includes internal splines 256adapted for meshed engagement with an externally-splined component ofcoupling 46. Obviously, similar splined inserts can be used inconjunction with rear output shafts 40, 40′ as well. An end cap 256 isshown to enclose the rear end of shaft segment.

Those skilled in the art can now appreciate from the foregoingdescription that the broad teachings of the present invention can beimplemented in a variety of forms. Therefore, while this invention hasbeen described in connection with particular examples thereof, the truescope of the invention should not be limited since other modificationswill become apparent to the skilled practitioner upon a study of thedrawings, specification and the following claims.

What is claimed is:
 1. A transfer case, comprising: a one-piece housingdefining first and second apertures and an opening; a first cover plateenclosing said first aperture of said housing and defining an opening; asecond cover plate enclosing said second aperture of said housing anddefining an opening; an input shaft extending through and rotatablysupported in said opening in said first cover plate; a first outputshaft driven by said input shaft and extending through and rotatablysupported in said opening in said housing; a second output shaftextending through and rotatably supported in said opening in said secondcover plate; and a mode clutch for transferring drive torque from saidfirst output shaft to said second output shaft.
 2. The transfer case ofclaim 1 wherein said first output shaft is tubular and includes a firstshaft segment and a second shaft segment, said first shaft segmentsupported by said input shaft and said second shaft segment extendingthrough said opening in said one-piece housing.
 3. The transfer case ofclaim 2 wherein said first shaft segment has a smaller outer diameterthan said second shaft segment.
 4. The transfer case of claim 3 whereinsaid first output shaft is formed to include a tapered transitionsegment interconnecting said first shaft segment to said second shaftsegment.
 5. The transfer case of claim 2 further comprising: a reductionunit having an input member driven by said input shaft and an outputmember driven at a reduced speed relative to said input member; a rangeclutch for selectively coupling one of said input member and said outputmember of said reduction unit to said first output shaft; and a shiftmechanism for controlling actuation of said range clutch.
 6. Thetransfer case of claim 5 wherein said range clutch includes a rangesleeve that is mounted on said second shaft segment of said first outputshaft for rotation and axial movement between first and secondpositions, said range sleeve operable in its first position to couplesaid input member of said reduction unit to said first output shaft andis further operable in its second position to couple said output memberof said reduction unit to said first output shaft.
 7. The transfer caseof claim 1 wherein said second output shaft is a tubular member having afirst shaft segment rotatably supported by said housing and a secondshaft segment rotatably supported by said second cover plate andextending through said opening in said second cover plate.
 8. Thetransfer case of claim 7 wherein said second output shaft furtherincludes a sprocket segment formed integrally between said first andsecond shaft segments.
 9. The transfer case of claim 1 wherein saidfirst cover plate includes a plate segment interconnecting first andsecond hub segments, said opening through said first cover plateextending through said first hub segment, and wherein said second hubsegment is sealed against said one-piece housing.
 10. The transfer caseof claim 1 wherein said first output shaft comprises a formed tubedefining an interior cavity and a plurality of steps along a length ofsaid tube wherein a grain structure of said tube is generallycontinuous.
 11. The transfer case of claim 1 wherein said second outputshaft comprises a formed tube defining an interior cavity and aplurality of steps along a length of said tube wherein a grain structureof said tube is generally continuous.
 12. The transfer case of claim 11wherein said tube further defines an external sprocket.
 13. A transfercase comprising: a one-piece housing defining an aperture and a firstopening; a cover plate enclosing said aperture of said one-piece housingand defining a second opening; an input shaft extending through saidsecond opening in said cover plate; a tubular output shaft driven bysaid input shaft and having a first tubular shaft segment supported bysaid input shaft and a second tubular shaft segment extending throughsaid first opening in said one-piece housing; a second cover plateenclosing a second aperture defined by said one-piece housing and havinga third opening; a second output shaft extending through said thirdopening in said second cover plate, said second output shaft is atubular member having a first shaft segment rotatably supported by saidhousing and a second shaft segment rotatably supported by said secondcover plate and extending through said third opening in said secondcover plate; and a mode clutch for coupling said second output shaft tosaid first output shaft.
 14. The transfer case of claim 13 wherein saidfirst shaft segment has a smaller outer diameter than said second shaftsegment.
 15. The transfer case of claim 14 wherein said output shaft isformed to include a tapered transition segment interconnecting saidfirst shaft segment to said second shaft segment.
 16. The transfer caseof claim 13 further comprising: a reduction unit having an input memberdriven by said input shaft and an output member driven at a reducedspeed relative to said input member; a range clutch for selectivelycoupling one of said input member and said output member of saidreduction unit to said output shaft; and a shift mechanism forcontrolling actuation of said range clutch.
 17. The transfer case ofclaim 16 wherein said range clutch includes a range sleeve that ismounted on said second shaft segment of said output shaft for rotationand axial movement between first and second positions, said range sleeveoperable in its first position to couple said input member of saidreduction unit to said output shaft and is further operable in itssecond position to couple said output member of said reduction unit tosaid output shaft.
 18. The transfer case of claim 13 wherein said secondoutput shaft further includes a sprocket segment formed integrallybetween said first and second shaft segments.
 19. The transfer case ofclaim 13 wherein said first output shaft comprises a formed tubedefining an interior cavity and a plurality of steps along a length ofsaid tube wherein a grain structure of said tube is generallycontinuous.
 20. The transfer case of claim 13 wherein said second outputshaft comprises a formed tube defining an interior cavity and aplurality of steps along a length of said tube wherein a grain structureof said tube is generally continuous.
 21. The transfer case of claim 13wherein said first cover plate includes a plate segment interconnectingfirst and second hub segments, said opening through said first coverplate extending through said first hub segment, and wherein said secondhub segment is sealed against said one-piece housing.